A trash pump is used for draining excavations and for similar pumping tasks in which it is required to pump water that may carry a significant load of solid particles such as sand. Because the solid particles would foul or ruin a positive displacement pump, a trash pump usually has a rotating impeller with vanes that are arranged to produce either an axial flow of the pumped liquid through the impeller, a radial flow, or a so-called mixed flow in which the impeller discharges the liquid generally radially but with a substantial component of rearward flow. An axial flow trash pump tends to operate at a relatively high output volume but with a relatively low pressure head, whereas a radial flow pump tends to discharge the pumped liquid under relatively high pressure but at a low rate. Thus, other things being equal, an axial flow trash pump is mainly suitable only for draining relatively shallow excavations, whereas a radial flow pump can drain a relatively deep excavation but does so rather slowly. A mixed flow pump offers a desirable combination of reasonably high volume and reasonably high pressure head, so that it is suitable for most applications, but heretofore mixed flow trash pumps have presented maintenance problems and have had other disadvantages which apparently discouraged their use to such an extent that axial flow and radial flow trash pumps have been commercially dominant.
In any trash pump, the vanes of the impeller move in an orbit that carries them very close to a fixed liner but not in contact with it. The smaller the distance between the vanes and the liner, the greater the efficiency of the pump, because the proximity of the vanes to the liner acts as a kind of seal between the high pressure and the low pressure sides of the impeller, preventing back flow to the extent that the seal is effective. On the other hand, the vanes cannot pass too close to the liner because solid particles carried by the pumped water have to be able to work through the space between the vanes and the liner in order not to jam the pump.
After a trash pump has been in use for some time, the abrasive action of water-carried particles tends to increase the distance between the vanes and the liner to the point where the efficiency of the pump is markedly impaired and repairs are needed. Since abrasive wear is practically inevitable in a trash pump, the facility with which repairs can be made is an important criterion of the merit of the pump.
In some prior trash pumps it was the front edges of the impeller vanes that passed close to the liner, and abrasive wear was compensated for by inserting shims behind the impeller to establish it at a more forward location on its shaft. Installation of the shims required disassembly of the pump to the point where the impeller was accessible so that it could be removed from its shaft. Opening up the housing of such a prior pump was seldom easy, but removal of the impeller was often a particularly difficult and annoying task because the impeller conventionally has a threaded securement to its shaft and the threads of the securement tend to become coated with sticky mineral and other deposits left behind by dirty water passing through the pump.
An old U.S. patent to Wilberforce, U.S. Pat. No. 640,345, issued in 1900, disclosed a radial flow trash pump having rigid liner elements that were spring urged into actual engagement with the front edges of the impeller vanes. The yielding mounting of the liner prevented solid particles from jamming the pump by facilitating their passage through it. Obviously, however, the arrangement produced relatively high frictional losses and must also have caused rather fast wear of the impeller and/or the liner.
U.S. Pat. No. 3,183,841, to Gaynor, disclosed a radial flow trash pump wherein the impeller had an annular lateral wall surface facing the inlet in the pump housing, and a resilient annular sealing member was secured in the housing in surrounding relation to the inlet and was maintained compressively engaged with the lateral wall surface on the impeller. To lubricate the zone of engagement between the resilient sealing member and the lateral wall surface on the impeller, clean water from an independent pressurized source was introduced through auxiliary water inlets in the housing. The utility of this pump was obviously limited to locations where pressurized clean water was available, which is to say that it could not be used in many situations where a trash pump was urgently needed.
More recently, U.S. Pat. No. 4,202,654, issued to A. S. Marlow in 1980, disclosed a radial flow trash pump having resilient annular liner elements adhesively secured to the front and back walls, respectively, of its volute, in coaxial relation to its impeller. The rear liner element lay closely behind a disc-like body or shroud plate of the impeller; the front one surrounded an inlet opening in the front wall of the volute and was close to the front edges of the impeller vanes, which projected forwardly from the shroud plate. To protect the impeller from erosion, it had an outer coating of elastomeric material. Although this coated impeller and its adjacent resilient liner elements were probably subject to less abrasion than more conventional all-metal parts, wear on those parts of a trash pump is practically inevitable, and replacement of those parts required a substantially complete disassembly of the pump for removal of the impeller from its threaded shaft. Since the pump comprised an extra-ordinarily large number of components, held together by bolts, its disassembly and reassembly tended to be complicated and time consuming in themselves.
As will be apparent from this rather sketchy summary of the state of the art, there has been a long-standing unfulfilled need for a trash pump that meets all of the following requirements:
The pump should be low in first cost but efficient, and should be versatile in having the capability for sustained operation without need for a supply of clean water; PA1 Although there should be only a small space between the impeller blades and the liner, fairly large water-borne particles should nevertheless be able to pass through that space without jamming the pump; PA1 The pump should sustain little abrasive wear on the opposing surfaces of its impeller vanes and liner during the course of a long period of operation; PA1 By means of simple shims it should be possible to compensate for such wear as does occur on the impeller vanes and/or liner; PA1 Shimming or replacement of the parts that are subject to abrasive wear should be possible with a minimum of disassembly and reassembly of the pump parts; and PA1 There should be no need to remove the impeller from its threaded shaft in order to compensate for abrasive wear, and the impeller should not normally require replacement during the life of the pump.